1. Field of the Invention
The present invention relates to a heat-resisting ohmic contact formed on a semiconducting diamond layer which is used for electronic parts or electronic devices such as diodes, transistors, FETs and thermistors, and to a process for forming the same.
2. Description of the Related Art
Diamond is an electrically insulating material having high hardness, and excellent thermal and chemical stability and radiation hardness. It has been studied with a view to, and has been adopted in various industrial applications. Meanwhile, diamond has a wide band gap of about 5.4 eV, and exhibits semiconducting properties upon doping with impurities such as Boron (B). The semiconducting diamond thus doped with impurities is expected to be applicable for high temperature use.
Methods of diamond films having such excellent properties using plasma reaction in vapor phase have been established. These have promoted the development of devices incorporating diamond films, for example, tool coating, speaker diaphragms, heat sinks for semiconductor devices, and electronic devices such as diodes and transistors.
In the electronic devices using semiconducting diamonds, ohmic contacts which are excellent in adhesiveness with the semiconducting diamond layers, that is, contacts with a linear current-voltage characteristic, must be formed. Taking into account the characteristic use of diamond at high voltages and high temperatures, the contact is required to be small in its contact resistance, and to be stable at temperatures higher than the service temperature. This is because a high contact resistance tends to cause the generation of heat at the contact area, resulting in a further localised increase in temperature.
Methods of fabricating ohmic contacts on semiconducting diamond layers have been proposed by A.T. Collins et al. (Diamond Research, pp. 19-22. 1970) and K.L. Moazed et al. (J. Appl. Phys., vol. 68, No. 5, pp. 2246-2255, 1990). Namely, as described in these references, there has been known a method of obtaining an ohmic contact by forming a layer of Ta, Ti, Mo or other element liable to form a carbide, followed by electron beam irradiation or vacuum heat treatment.
The above method is carried out as follows: Namely, as shown in FIG. 4a, a carbide layer 2 is formed at an interface between a diamond layer 3 on a substrate and a metal contact layer 1 made from Ta, Ti, Mo or the like, to thus obtain a preferable ohmic contact. In other words, a metal such as Ta, Ti, Mo or the like is used as a contact material, and the carbide layer 2 is formed between the contact layer 1 and a diamond layer 3.
On the other hand, in recent years, a method using a multi-layer metal film has been reported wherein Ti, which forms the most stable carbide TiC.sub.1-x (where 0&lt;.times.&lt;1) is used as the contact material, and Au, Pt or Mo/Au (Mo and Au are layered in this order) is formed on the Ti contact for preventing the oxidation of Ti.
In an ohmic contact shown in FIG. 4b, a contact Ti layer 1 is formed on a substrate composed of a semiconducting diamond layer 3 and a diffusion preventive Au layer 4 is formed on the contact Ti layer 1.
In an ohmic contact shown in FIG. 4c, a diffusion prevention Mo layer 5 is formed on a contact Ti layer 1 on a semiconducting diamond layer 3, and further a diffusion prevention Au layer 4 is formed on the diffusion prevention Mo layer 5. The semiconductor devices using this multi-layer contact of Ti/Mo/Au include thermistors etc. (Fujimori et al., NEW DIAMOND Vol. 13, P32, 1989).
Ti is generally used as the material for an ohmic contact: however, it has a high chemical reactivity and is liable to be oxidized at high temperatures in air. Accordingly, as described above, for preventing the oxidation of Ti, a multi-layer film including Au, Pt, Mo/Au or the like as an oxidation prevention layer is used.
With use over a long period, however, it is difficult to perfectly shield the contact from oxygen. Actually, under the condition shown in Table 1, the contact composed of a contact Ti layer 1/diffusion prevention Au layer 4 shown in FIG. 4b was fabricated, and was examined for oxidation prevention performance.
FIG. 5a shows the distribution of each element in the depth direction as measured by X-ray photoelectron spectroscopy. In this graph, the ordinate indicates a ratio of the atomic concentration of each element (atomic %), and the abscissa indicates the sputtering time corresponding to the distance from the surface of the contact. As can be seen in FIG. 5a, as one moves from the surface of the contact, the layers are arranged in the following order; Au, Ti, diamond (C).
The contact was next kept for 60 min at 500.degree. C. in air, which gave the result shown in FIG. 5b. As can be seen in FIG. 5b, Ti had diffused to the surface and oxidized to form TiO.sub.2. The reason for this is as follows: namely, the carburization of Ti is started on the surface of the diamond by annealing of the sample, when forming the ohmic contact; however, at the same time, the contact material Ti diffuses through the Au layer 4 formed on the Ti layer and reacts with the oxygen on the surface of the contact, and is oxidized.
TABLE 1 ______________________________________ formation condition Ti layer Au layer ______________________________________ DC discharge condition 0.8 A, 380 V 0.2 A, 510 V film deposition time 15 sec 1 min film thickness 400 .ANG. 2000 .ANG. gas pressure 2 mTorr discharge gas Ar ______________________________________
Next, a Ti/Mo/Au contact, in which an Mo layer 5 for preventing the diffusion of Ti was inserted between a Ti layer 1 and an Au layer 4, was fabricated (see FIG. 4c). Table 2 shows the film formation conditions. Further, FIG. 6a shows the concentration distribution of elements in the depth direction as measured by X-ray photoelectron spectroscopy. In this graph, the ordinate indicates the ratio of the atomic concentration of each element (atomic %), and the abscissa indicates the distance from the surface of the contact. The contact thus fabricated was next kept for 60 min at 500.degree. C. in air, as a result of which the concentration distribution of each element changed as shown in FIG. 6b.
As shown in FIGS. 6a and 6b, even in the Ti/Mo/Au contact using Mo as the diffusion prevention layer, the oxidization of Ti cannot be prevented.
TABLE 2 ______________________________________ formation condition Ti layer Mo layer Au layer ______________________________________ DC discharge 0.8 A, 380 V 0.2 A, 320 V 0.2 A, 510 V condition film deposition time 15 sec 1 min 1 min film thickness 400 .ANG. 400 .ANG. 2000 .ANG. gas pressure 2 mTorr discharge gas Ar ______________________________________